1. Field of the Invention
This invention relates in general to a transducer which operates with acoustic surface-proximate waves and comprises interdigitally arranged finger electrodes and bus bars mounted on piezoelectric material wherein the finger electrodes are essentially of the same width and have identical spacings from each other wherein the finger electrodes are arranged in groups relative to each other such that the neighboring groups are effective to cause an out-of-phase relationship of the surface waves.
2. Description of the Prior Art
Arrangements which operate with acoustic surface-proximate waves are known. Acoustic surface-proximate waves are waves which are referred to less precisely as surface waves, but which include not only surface waves in the stricter sense (SAW-waves; Rayleigh waves) but also include Bleustein waves, SSBW waves (surface skimming bulk waves), STW waves (surface transverse waves), Love waves. Such arrangements utilize transducers for converting electrical signals into such waves or, respectively, for converting such waves into electrical signals. Transducers are known comprising interdigital transducers which are formed of interdigitally arranged finger electrodes which usually are respectively connected to a bus bar. The basic type of such interdigital transducer is composed of two bus bars which extend parallel to each other from which extend rectangular narrow metallization strips are respectively directed toward the other bus bar and which are referred to as finger electrodes and are connected to the associated bus bar which extends on the surface of the substrate. In this basic type, neighboring finger electrodes which follow after each other are electrically connected alternately to the bus bars. Any arrangements of finger electrodes of a transducer of the basic type are also effectively equiphase inside the transducer.
The functioning of such a transducer is such that the acoustical (mechanical) wave is generated by the piezoelectric effect in the piezoelectric material between the finger electrodes and is generated by applying an electical signal between the two bus bars and the signal thus appears between the neighboring finger electrodes. Such an interdigital finger electrode arrangement has a fundamental resonant frequency which corresponds to the periodicity of the fingers. Such an interdigital transducer usually emits acoustic waves in the material in both directions perpendicular to the finger electrodes. Such an interdigital transducer however has the undesirable characteristic that reflections of the generated acoustic wave occur inside the transducer particularly at the finger electrodes. For this reason, the use of split finger electrode arrangements for transducers has already been provided for more than ten years. The utilization of such split finger arrangements however has the disadvantage in that the maximum obtainable fundamental resonant frequency is only one-half as high as that obtainable with a transducer of the basic type wherein the precondition of the finger electrodes are of equal width is maintained. The maximum possible fundamental frequency for such transducers is defined by the technological limit for the manufacture of electrode fingers having the smallest possible width. This dimension at the present time is about 1 .mu.m using a spacing about 1 .mu.m clearance between neighboring finger electrodes.
Publication IEEE Transactions On Sonics and Ultrasonics, Vol. SU-22 (1975), pages 395-401, shows in FIG. 3 transducer arrangements of various types. In FIG. 3, the uppermost example is a transducer arrangement of the basic type. The third view shows a split finger arrangement which has been described previously. The transducer of the second line illustrates an arrangement which in terms of principal is between the other two transducer arrangements. The fourth line shows a split finger arrangement comprising additionally inserted floating fingers which are fingers which are not connected to any of the bus bars, in other words, have a floating potential. The position of the resonant frequencies of the respective transducer is indicated to the right of these transducer arrangement in this FIG. 3. The fundamental resonant frequency of the associated transducer is listed with respect to line 1. The fundamental frequencies always become smaller and the associated higher harmonics or, respectively, harmonic frequencies are indicated in the lines which are below. The split finger transducer of line 3 has a fundamental frequency which is one-half as high as the transducer of line 1 and has a resonance at the third harmonic. Analogous situations applies to the embodiments illustrated in the remaining lines. However, it should be realized that with respect to FIG. 1 that operation of the respective transducer at its harmonic is not practical. It must also be realized that the further development of a transducer of line 1 in the direction of the transducers of the following lines 2, 3 and 4 of FIG. 3 in this publication results in an increasing reduction of the frequency which can be used as the fundamental frequency.
Another way of eliminating the internal reflections in an interdigital transducer of the basic type is to provide unequal clear spacings within the finger electrode arrangement of the transducer. Assuming that the finger width of such a transducer is .lambda./4 wherein .lambda. is the wavelength of the acoustic wave at the fundamental resonance and, thus, the narrow spacing between neighboring fingers is likewise selected to be equal to .lambda./4 then the broader clearance between electrode fingers in such transducers is .lambda./2 wherein destructive interferences for reflected portions of the acoustic wave are effected. With comparable finger width such a transducer has the same fundamental frequency as the transducer of the basic type. Such a transducer however has relatively pronounced secondary lobes which can result in disturbances. A further disadvantage of this transducer is the unequal spacing between electrode fingers. Unequal clearances lead to irregularities in the manufactured structure and when using the exposure process which is necessarily used to manufacture the structure this causes additional disturbances in the electrical characteristics of such a transducer.
Transducers manufactured and operated as code generators such as known from the two publications IEEE Transactions On Circuit Theory, Vol. CT-20, No. 5, September 1973, Pages 459-470 and particularly FIG. 13 and the publication by Matthews, entitled Surface Wave Filters, 1977, Pages 307-346, by John Wiley and Sons, particularly FIG. 7.1 have a purely accidental and only apparent similarity to the present invention. These articles disclose code generators which comprise finger groups which have an incorporated code on the basis of their different group lengths or, respectively, the number of fingers of the individual groups. For example, the finger group which is the first from the left in FIG. 13 therein has a length which is five times as great as the narrowest finger group. An identical number of fingers for all groups is impossible in such a code generator or the finger groups would have no informational content.
See also U.S. Pat. No. 4,506,239, 4,099,148, 3,766,496, 3,701,147, 3,633,118, 3,551,837 and publications U.S. Electronics Letters, 17 May 1973, Vol. 9, No. 10, pages 239-240 and U. S. Microwave Journal of July 1974, Pages 42-45.